Conventional optical semiconductor devices including an optical semiconductor element mounted therein have a configuration including a metallic lead frame 1, an optical semiconductor element 2 mounted thereon, and a reflector 3 for light reflection formed of a resin material so as to surround the periphery of the optical semiconductor element 2, as shown in FIG. 1. Then, a space including the optical semiconductor element 2 surrounded by the reflector 3 is encapsulated with a transparent resin such as a silicone resin. An electrode circuit (not shown) formed in the metallic lead frame 1 and the optical semiconductor element 2 are electrically connected with each other by a bonding wire 4 to be provided according to the need.
Now, an epoxy resin composition is used for encapsulating an optical semiconductor element such as a light emitting diode (LED). In particular, for epoxy resin compositions which are developed with a mind to the use in the surroundings of LED, a material including an aliphatic (alicyclic) hydrocarbon as a main structure is used as an epoxy resin capable of making the absorption in a visible light short wavelength region low (see Patent Document 1).    Patent Document 1: JP-A-2007-91960